The ever increasing density of semiconductor devices in integrated circuit manufacturing requires ever increasing precision in device manufacturing. Of particular importance is the ability to control the gate length in field effect transistors (FETs). Without the ability to shorten gate length, an increase in density and circuit performance could not be accomplished.
Current FET gate manufacturing methods result in non-uniform gate length, circuit damage from reactive ion etch (RIE) of the gate lines, and/or require additional processing steps. For example, a substitutional gate method of producing FET gates uses an oxide plug that is created using an anisotropic etch. The oxide plug is then coated with polymer and its surface is planarized. The plug is then removed and substituted by gate metal through lift-off techniques. There are several disadvantages in using the oxide plug method. First, it requires a large increase in photo and process steps. Second, spacers cannot be used in an oxide plug method to reduce gate size. Third, the lift-off technique severely limits the size and thickness of the gate metal line making the process virtually impossible for sub micron gate lengths.
Another method again uses an anisotropic dielectric etch process to control gate length by etching an opening instead of a plug. A dielectric is deposited then the gate opening is anisotropically RIE etched down to the substrate to define gate dimensions. The opening is filled with the gate material and the top surface of the gate line is then patterned with photoresist and etched. Again there are several disadvantages to this method. Since the gate is patterned and etched leaving full thickness on top of the remaining oxide, the planarity of the device is lost, requiring additional dielectric planarization techniques prior to routing interconnect metallization. In addition, spacer technology cannot be used. Without spacer technology, it is difficult to optimize for high device performance without introducing drain induced barrier lowering or device breakdown control problems. This limits the performance and scaleability of the device.
A third method for controlling gate length would be a low pressure, high plasma density, RIE etch tool which typically etches material in the five to ten millitor range. Under these conditions, an anisotropic etch of the gate material can be obtained. Again there are several disadvantages to this method. Damage and contamination from the gate material deposition and etch process can degrade device performance by creating surface states or destroying the crystal structure of the semiconductor. Also, the gate dimension cannot be reduced below the capability of the photoresist alignment tool since sidewall spacers cannot be used to shrink the gate opening beyond its resolution capability.
A damage and contamination free method of reducing gate length variability without an increase in processing steps that provides self-alignment and scaleability is needed.